1. Field of the Invention
This invention relates generally to copolymers having improved physical and chemical properties which are produced using a protecting group (adduct), particularly an alkyl boronic acid adduct of glyceryl methacrylate or glyceryl acrylate, in combination with an alkyl acrylate or alkyl methacrylate, and optionally using a crosslinking agent such as a glycidyl acrylate glycidyl methacrylate, polyol diacrylate or polyol dimethacrylate. The copolymer has improved handling, machining and shaping properties with respect to water sensitivity in comparison with related materials. Removal of the adduct, as by treatment with hot water or hydrogen peroxide buffer, produces a hydrogel which is useful as a contact lens material.
2. Prior Art
While glass contact lenses were used to correct vision problems in the early 1900's, widespread use and development of the contact lens field did not occur until the advent of organic polymer chemistry during and after World War II. During this period hard contact lenses were usually made from the polymer, polymethylmethacrylate. Some problems encountered with the use of this polymer as a contact lens material include:
(1) inability of many people to adapt to the presence of a hard lens in the eye;
(2) small particles lodged under the lens cause irritation of the cornea;
(3) after one to five years of use, many people experience discomfort and discontinue use; and
(4) impermeability of the polymer to physiological agents, such as oxygen and carbon dioxide necessary for corneal metabolism.
This last property causes "oxygen starvation" which produces "corneal edema," a serious eye condition which is usually reversible by discontinuing use of the hard contact lens.
In view of the above difficulties, efforts were made to develop new materials which overcome the problems of the hard lens. One class of such lens materials are hydrogels of sparingly cross-linked hydrophilic comonomers, and unsaturated organic acids and diacids, described for example in U.S. Pat. Nos. 2,976,576 and 3,220,960. A specific hydrogel disclosed in these patents is a slightly crosslinked material comprising a major amount of 2-hydroxyethyl methacrylate. This hydrogel has become widely known in the art as "HEMA" and is particularly useful because of its ability to absorb about 35 to 45 percent by weight of water. The water renders the lens permeable to oxygen, and also soft and flexible so as to enable it to conform to the curvature of the eye.
In spite of its obvious advantages, the HEMA lens presented some additional problems, including:
(1) a lack of clarity of central vision because of a constantly changing optical surface resulting from normal eye movement and blinking;
(2) a lack of sufficiently rigid, non-deforming surface which would correct astigmatism;
(3) corneal irritation possibly caused by a lack of tear exchange because the tight lens conformation to the eye prevents flow at the edge of the lens;
(4) an accumulation of catabolic products; and
(5) a lack of structural stability.
Additional polymers and manufacturing techniques to improve the overall properties of contact lenses are disclosed in U.S. Pat. Nos. 3,361,858; 3,542,097; 3,647,736; 3,700,761; 3,728,315; 3,926,892; 4,045,547; 4,095,877; 4,109,070; and 4,113,686.
A new contact lens material has been reported to overcome the problems described above and is disclosed in U.S. Pat. No. 4,056,496, which is incorporated herein by reference. These materials are hydrogels formed from a dihydroxyalkyl acrylate or methacrylate, an alkyl acrylate or methacrylate and optionally a minor amount of an epoxidized alkyl acrylate or methacrylate. The properties of these hydrogels when compared to HEMA are:
(1) stronger and stiffer;
(2) capable of being fabricated of a thinner cross-section;
(3) capable of providing increased pumping action of tears caused by blinking;
(4) substantial oxygen and carbon dioxide permeability needed for normal corneal metabolism;
(5) extended wear--in some cases up to six months;
(6) improved ability to be cleaned; and
(7) a toughness which prevents tearing.
The polymers described in U.S. Pat. No. 4,056,496 on occasion were found to present problems during shaping and machining, probably due to the polymer's ability to absorb water from the atmosphere. Careful control of the humidity during machining was found to be necessary. Further, water based or compatible machining and polishing oils and abrasives could not be used because of their incompatibility with the polymer. Finally, polymers of this type often become seriously distorted after machining when they absorb water necessary to be useful as contact lens materials. Correction of this lens distortion is expensive, time-consuming, and sometimes impossible (resulting in the total loss of previously manufactured polymer material).
It would, therefore, be useful to have a polymeric material which could be easily machined using water based and compatible oils, polishes and abrasives without regard to humidity. This polymer could subsequently be treated to produce a hydrogel material having the desired properties, e.g., minimal distortion, for use as a contact lens material.